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Numerical Simulation for Operation of Flexible Thin-Film Transistors With Bending
Asadirad, Mojtaba,Pouladi, Sara,Shervin, Shahab,Oh, Seung Kyu,Lee, Keon Hwa,Kim, Jeomoh,Lee, Sung-Nam,Gao, Ying,Dutta, Pavel,Selvamanickam, Venkat,Ryou, Jae-Hyun IEEE 2017 IEEE electron device letters Vol.38 No.2
<P>We theoretically study the change of the performance characteristics with various mechanical bending conditions for flexible thin-film transistors (TFTs) by two-dimensional device simulation. The characteristics of newly developed flexible TFTs with high crystalline quality and high carriermobility aremore sensitive to the degree of bending. We developed a model to estimate the change in the characteristics as a function of curvature radius of the channel with a focus on scattering of carriers in a bent TFT. Field-effectmobility decreases by bending, e.g., similar to 11% with a radius at R = 30 mm and a knee voltage increases, while a threshold voltage remains the same. This model can be extended to other flexible TFTs with bending.</P>
Shervin, Shahab,Oh, Seung Kyu,Park, Hyun Jung,Lee, Keon-Hwa,Asadirad, Mojtaba,Kim, Seung-Hwan,Kim, Jeomoh,Pouladi, Sara,Lee, Sung-Nam,Li, Xiaohang,Kwak, Joon Seop,Ryou, Jae-Hyun Institute of Physics Publishing Ltd. 2018 Journal of Physics. D, Applied Physics Vol.51 No.10
<P>We report a new route to improve quantum efficiencies of AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) using mechanical flexibility of recently developed bendable thin-film structures. Numerical studies show that electronic band structures of AlGaN heterostructures and resulting optical and electrical characteristics of the devices can be significantly modified by external bending through active control of piezoelectric polarization. Internal quantum efficiency is enhanced higher than three times, when the DUV LEDs are moderately bent with concave curvatures. Furthermore, an efficiency droop at high injection currents is mitigated and turn-on voltage of diodes decreases with the same bending condition. The concept of bendable DUV LEDs with a controlled external strain can provide a new path for high-output-power and high-efficiency devices.</P>
Lee, Keon Hwa,Park, Hyun Jung,Kim, Seung Hwan,Asadirad, Mojtaba,Moon, Yong-Tae,Kwak, Joon Seop,Ryou, Jae-Hyun Optical Society of America 2015 Optics express Vol.23 No.16
<P>We study light-extraction efficiency (LEE) of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs) using flip-chip (FC) devices with varied thickness in remaining sapphire substrate by experimental output power measurement and computational methods using 3-dimensional finite-difference time-domain (3D-FDTD) and Monte Carlo ray-tracing simulations. Light-output power of DUV-FCLEDs compared at a current of 20 mA increases with thicker sapphire, showing higher LEE for an LED with 250-관m-thick sapphire by ~39% than that with 100-관m-thick sapphire. In contrast, LEEs of visible FCLEDs show only marginal improvement with increasing sapphire thickness, that is, ~6% improvement for an LED with 250-관m-thick sapphire. 3D-FDTD simulation reveals a mechanism of enhanced light extraction with various sidewall roughness and thickness in sapphire substrates. Ray tracing simulation examines the light propagation behavior of DUV-FCLED structures. The enhanced output power and higher LEE strongly depends on the sidewall roughness of the sapphire substrate rather than thickness itself. The thickness starts playing a role only when the sapphire sidewalls become rough. The roughened surface of sapphire sidewall during chip-separation process is critical for TM-polarized photons from AlGaN quantum wells to escape in lateral directions before they are absorbed by p-GaN and Au-metal. Furthermore, the ray tracing results show a reasonably good agreement with the experimental result of the LEE.</P>
Seung Hwan Kim,Singh, Shivkant,Seung Kyu Oh,Dong Kyu Lee,Keon Hwa Lee,Shervin, Shahab,Asadirad, Mojtaba,Venkateswaran, Venkat,Olenick, Kathy,Olenick, John A.,Sung-Nam Lee,Joon Seop Kwak,Mavrokefalos, IEEE 2016 IEEE electron device letters Vol.37 No.5
<P>We demonstrate flip-chip light-emitting diodes (FC-LEDs) on a flexible yttria-stabilized zirconia (YSZ) substrate and compare them with FC-LEDs on a polymeric substrate. Degradation of luminescence intensity and red-shift of peak wavelength are not observed for the LED on the flexible YSZ, unlike one on the polyimide substrate, due to improved capability to remove the generated heat from the chip to the substrate. Thermal distribution measurements and finite-element simulations show improved thermal management by the flexible ceramic as compared with previously developed flexible LEDs on polymeric substrates. The results present an improved solution to high power operation of flexible LEDs.</P>